1. Field of the Invention
The present invention relates to a device for controlling a behavior or an attitude of a vehicle such as an automobile, and more specifically, to such a device modified against a highly unstable behavior such as when a risk of rolling over a vehicle body is detected.
2. Description of Prior Art
A modern vehicle employs a system for controlling a behavior or an attitude of a running vehicle body. In such a vehicle, for instance, a braking system, in which braking force on each wheel can be independently increased and decreased, adjusts braking and/or traction force on wheels to modulate yaw moment and centripetal force on the vehicle body for maintaining a dynamical stability of the running vehicle under the control of an electronic control device. In accordance with such a system, which is often called “VSC (Vehicle Stability Control)” system, spinning (oversteering) and/or drifting-out (understeering), caused by excessive yaw moment around the centroid of a vehicle body and/or saturation of road reaction force (traction/braking force) on wheels, are effectively suppressed. So far, behavior control devices have been also developed against highly unstable or deteriorated behaviors, such as rolling over a turning vehicle, caused by abrupt steering or excessively high speed turning on a highly frictional road surface. An example of such devices is seen in Japanese Patent Laid-open Publication No. 11-11272, in which, in response to a detection of a risk of rolling-over based upon speed, steering angle and roll rate of a turning vehicle, the vehicle is braked to reduce centrifugal force exerted on its body, and thereby the risk of rolling-over can be decreased.
In conventional control strategies for vehicle behavior as described above, transient variation of braking force during correcting and maintaining a vehicle behavior is less considered. During controlling a vehicle behavior (a feedback control is usually employed for rendering an actual value in conformity with the corresponding target control amount.), braking force values or slip ratios on wheels are rendered close to their respective target amounts ensuring the maintaining of vehicle stability by cyclically incrementing or decrementing the braking force values. However, an increment and/or a decrement, i.e. a correction amount, in one cycle and/or a feedback gain are rather roughly determined. The behavior would be more deteriorated transiently during the control process when a correction amount applied to the corresponding control object is too large.
In this regard, when highly deteriorated condition, leading to the rolling-over of a vehicle body, is detected, it is possible that relatively strong force components, i.e. centrifugal force; yaw, roll and pitch moments, etc. have been already exerted on the vehicle body. In order to correct such a highly deteriorated behavior while reducing the risk of rolling-over, a rather large control amount is required against strong force and moment already exerted. However, such a rather large control amount induces a large variation of braking force on wheels, often resulting in the increase of rolling and pitching oscillations of a vehicle body.
Accordingly, a vehicle behavior control device may be improved, while taking into account transient braking force variation during control processes of correcting and maintaining of a vehicle behavior, especially under condition that a large control amount for correction is required against a highly deteriorated behavior, such as when a risk of rolling-over of a vehicle is detected. In this connection, since it becomes more difficult to correct a vehicle behavior having been already highly deteriorated (e.g. just before the rolling-over becomes inevitable), it is preferable to predict a future risk of behavior deterioration, and to correct the behavior in advance, thereby effectively reducing a risk of falling into ultimate deterioration, such as rolling-over of a vehicle body.